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Compressor suction drums

T his happens in petrochemical plants, ren-

eries and anywhere else where the gas

approaching a compressor is wet. Traces of

aqueous or organic liquid escape the inlet knock-

out drum, often intermittently, and silently

damage the compressor. Telltale signs include

pitting corrosion, salt deposits and diluted

lubricants.

Instead of trying to repair symptoms, look forthe root cause, which usually involves the mist

eliminator in the knockout drum (see Figures 1

and 2). Problems may include improper mist

eliminator specications, overloading, uneven

velocity proles, incorrect installation, high

liquid viscosity, waxy deposits, liquid slugs,

foaming and several other possibilities.

The trouble may even be that no mist elimina-

tor was provided in the rst place, or perhaps no

knockout drum at all. But wherever free liquid

drops out in a suction drum, it generates somemist that can damage the compressor unless it is

removed by a mist eliminator. Even in cases

where the feed gas never has any free liquid,

there are often ne mist droplets that coalesce

into large drops on the walls of the inlet pipe or

inside the compressor. For all but the driest gas,

a compressor should be protected by an inlet

mist eliminator. New high-capacity, high-

efciency mist eliminator technologies pay off

the rst time you avoid a shutdown.

For optimum separation performance,compressor knockout drums must be properly

designed and sized with appropriate mist elimi-

nator elements in correct congurations, taking

into account many factors. In multistage

compressor installations, the proper knockout

drum design is seldom the same for all stages.

To maintain good performance, the design of

each drum should be reviewed whenever there

are signicant changes in the process, such as

increases or decreases in throughput, shifts in

Amistco

composition of the gas or mist droplets, altera-

tions in upstream equipment, or revisions to

operating and control procedures. In addition,

mist eliminator elements should be visually

inspected occasionally (especially after major

process upsets) to make sure they are intact and

free of excessive solid deposits.

A thorough understanding of the relevant

considerations will help you avoid common

www.digitalrefining.com/article/1000038 May 2004 1

I think I have got liquid carryover. What can I do about it?

Figure 1 Typical compressor suction drums

Figure 2 Typical multistage compressor installation

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suction-drum pitfalls and some not-so-commonones that could severely damage your compres-

sors due to liquid carryover. For detailed

explanations of mist eliminator selection, sizings

and vessel design for a wide range of applica-

tions, see Amistco’s literature such as Amistco

Mesh & Vane Mist Eliminators, Bulletin 106.

This paper provides information that applies

specically to compressor inlet knockout drums.

Designing for droplet size distribution

There are many different types of mist elimina-tor elements, and the variety has greatly

increased through the years. Not understanding

the liquid source in the upstream process can

cause you to select the wrong type of mist elimi-

nator, or to keep a given type when process

changes make it inappropriate.

Understanding the process allows you to

design for the most efcient mist collection.

Most important, selection should not be made

2 May 2004 www.digitalrefining.com/article/1000038

until the droplet size distribution is dened, in

terms of the proportion of droplets of each size.

Assuming an incorrect droplet size distribution

can mean that you have designed for a less ef-

cient mist eliminator and liquid carryover may

occur.

See Tables 1 and 2 for some points of refer-

ence and rough guidelines in this respect. Be

aware that the capture efciency of a given mist

eliminator element does not depend only ondroplet size. It is also inuenced by gas velocity

through the element and mist load in terms of

liquid ow rate per unit of cross-sectional area.

Then there are variables such as density and

viscosity that depend on temperature, pressure,

and liquid and gas composition. All else being

equal, efciency generally goes up with higher

velocity, ner mesh strands, closer packing of

mesh (greater density), closer spacing of vanes,



Tables 1 and 2

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and greater thickness of the mist eliminator

element.

Mesh pad fouling In some cases, liquid carryover to the compres-

sor is caused by fouling of a mesh-type mist

eliminator, on account of the resulting restric-

tion to gas ow and extra holdup of liquid in

the pad. Vane-type mist eliminators are a better

choice for fouling applications. Due to the rela-

tively wide open spaces between blades, vanes

are much less likely to plug. If the fouling

deposit can be readily dissolved by a suitable

solvent, as might be the case with viscous oils

or waxes or certain caked solids, consider

installing a spray system, as shown in Figure 3,

to clean the vanes on-line whenever necessary.

Adding a high-efciency mesh mist eliminator

downstream of the vane unit (also shown

in Figure 3) can help make up for theinherently lower droplet capture efciency of

the vanes. Figure 5 Typical multistage compressor installation






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6 illustrates guidelines for sufcient distance

between the mist eliminator and the gas inlet

and outlet.

If spacing is too close, the gas will pass though

only part of the mist eliminator. This causes

localised high velocities with liquid re-entrain-

ment and low velocities with poor efciency, as

shown in Figure 7.

In an existing conventional drum where the

mist eliminator is too close to the gas outlet,

there may not be enough room to lower the mist

eliminator. The solution then might be a prop-

erly designed ow distribution device located

above the mist eliminator to create a more

uniform velocity prole.

Overcoming pressure-drop constraintsProcesses that operate under vacuum or very low

pressure immediately upstream of the compres-

sor can be very tricky for suction drum design, because the pressure drop across the mist elimi-

nator must be kept low. However, generally

speaking, the lower the pressure-drop character-

istics of a mist eliminator type, the lower its

efciency in removing mist. Liquid carryover

from the suction drum may be a result of select-

ing a low-efciency mist eliminator for the sake

of low pressure drop.

When high efciency is not required, a vane

unit or low-density mesh pad (Amistco TM-1105)

may be recommended to achieve low pressuredrop. To gain higher efciency without much

cost in terms of pressure drop, a possible solu-

tion is a dual-density mesh pad. In such a pad,

the downstream layer has higher density than

the upstream layer. The result is higher separa-

tion efciency with only a slight increase in

pressure drop.

Throughput exceeding design capacity

In designing compressor knockout drums, like

other gas liquid separator vessels, conventionalmist eliminators are generally selected and sized

with a margin of about 10% above the design

throughput. Flow rates beyond the upper operat-

ing limit may allow liquid carryover due to high

velocities that cause re-entrainment from the

mist eliminator element.

More specically, mist eliminators are typically

sized for a cross-sectional area to achieve a

design velocity according to the Souders-Brown

vapour load factor K:

Liquid slugs and high liquid loading

In some applications, liquid slugs occasionally

come in with the gas feed. These surges can

temporarily overwhelm the slug-catching capa-

bility of the inlet knockout drum and ood a

mesh-type mist eliminator, causing liquid carry-over (see Figure 4).

When liquid slugs or generally high liquid

loading are expected, it is recommended to use a

vane-type mist eliminator upstream of the mesh

pad, as shown before in Figure 3. Vanes can

generally handle up to 10 times more liquid load

than mesh pads.

Breaking inlet foam

If liquid in the gas approaching the compressor

knockout drum is subject to foaming, it canreadily ood a mesh pad and, in severe cases,

even a vane unit. The end result is massive liquid

carryover from the vessel and damage to the

compressor. A vortex-tube cyclone device (see

Figure 5) can break up foam in the incoming

feed.

Dealing with high liquid viscosity

There are applications in which high viscosity

impedes liquid drainage so severely that a mesh

pad would ood at prohibitively low velocitiesand liquid loading. In these applications, a vane

unit is the better choice. For high efciencies,

consider Amistco’s double-pocket vanes, which

handle high-viscosity liquids with an efciency

of 99.9% for 8-micron and larger droplets.

Mist eliminator spacing in the vessel

An often overlooked but very important aspect

of suction drum design that can lead to liquid

carryover is proper spacing in the vessel. Figure



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VG = Gas velocity (volume ow divided by

cross-section)

PL = Liquid density

PG = Gas density

Conventional horizontal mesh pads are tradi-

tionally sized for a K factor of 0.35 ft per second,

which corresponds to a velocity of 10 ft per

second in the reference case of water and air at

room conditions. When a knockout drum’s throughput has

grown to exceed its capacity, there are generally

two options:

• Replace the vessel with a larger one to allow a

mist eliminator with greater cross-sectional area,

thus reducing the velocity

• Replace the mist eliminator in the existing

vessel with one using the latest technology to

maintain efciency with higher throughput.

Option 2 is generally much more cost-effective

and often does not require prohibitive downtime.In a traditional vertical cylindrical vessel, the

traditional horizontal orientation is no longer the

only solution. Compressor knockout drums can

be retrotted for capacity increases using any of

the following techniques:

• Vertical mist eliminator elements with hori-

zontal ow (K= 0.42 for mesh pads, K = 0.65 for

vane units)

• Properly engineered bafing for even velocity

proles with close spacing

• Horizontal mesh pads with drainage layers or

multiple zones that can increase capacity by 10-

12% (K = 0.40)• Amistco double-pocket vanes that can double

the capacity of a conventional vane unit (K =

0.8–1.1)

• Mesh-vane combinations that can increase

efciency and capacity by 10–25% (K =

0.5–0.65)

• Mesh agglomerator followed by double-pocket

vanes for highest efciency (99.9% of 2-micron

droplets) and greatest capacity increase

• Two- or four-bank congurations that allow








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mist eliminator elements of greater cross-

sectional areas.

Figure 8 illustrates several of these possibilities:

horizontal ow through vertical mist eliminator

elements, use of mesh pads to agglomerate ne

mist into large droplets that are removed by vaneunits, and a double-bank conguration. Amistco’s

design specialists can help apply such advanced

means of optimising the efciency and capacity of

existing knockout drums to create optimal solu-

tions for particular applications.

Retrofit without recertifying for ASME codeRetrotting an existing vessel for any of the fore-

going suggested remedies has one drawback: it

often requires the welding of new support rings,

beams, clips and other structures to the vessel wall. Most vessels are ASME code certied. Thus,

after welding to the vessel wall, the welded area

must be heat-treated, and the vessel must be

recertied. It is generally desirable to avoid this

cumbersome procedure. Amistco offers expan-

sion rings that are made in sections that can be

passed through a manway (see Figure 9).

The rings are then bolted together and wedged

against the vessel wall without welding. The

unique double expansion design ensures that the

installed ring does not move during operation.

Once the rings are installed either in vertical or

horizontal vessels, beams can be bolted to the

rings and complete housings can be built up

inside the vessel.

Using inlet diffusers to relieve carryover

Inlet design is one of the most commonly

neglected aspects of a compressor knockout

drum design, thus often the cause of poorperformance. In the example shown in Figure

10, a half-pipe inlet deector projecting into the

vessel reduces the gas ow area of that position,

with the following results:

• Gas jets to the back wall of the vessel

• Without enough space to diffuse the jet, gas

utilises only part of the mist eliminator. Due to

an uneven velocity prole, liquid carryover

occurs, as in Figure 7

• The gas jet agitates the accumulated liquid

below, generating droplets• Turbulence spoils normal gravity settling of

larger liquid droplets below the mist eliminator.

Additional liquid load increases the likelihood of

ooding the mist eliminator.

A properly selected inlet diffuser added to an

existing knockout drum (see Figure 11) provides

more effective separation of liquid coming in

with the gas and distributes the gas evenly

throughout the vessel diameter before the gas

moves upwards.




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Damage by sudden pressure changes

Carefully review the transient pressures that

occur at the knockout drum and mist eliminator

during all phases of operations. The suction

drum could see a sudden surge of ow in either

direction due to compressor recycle or opening

an anti-surge valve.

Pads and vanes can also be damaged by freez-

ing liquids in cold climates. In natural gas

production and pipelining, hydrate formation is

known to destroy mesh pads and vanes.

To cope with any of these scenarios, as shown

in Figure 12, any or all of the following remedies

can be applied to mist eliminator elements:

• Reinforce with heavy-duty material

• Fasten with bolts instead of traditional tie

wires, or provide an upper support ring in addi-

tion to the lower one

• Provide a pressure relief door in the case of

mesh.

Compressor knockout drum casesEthylene plant debottleneck

A large ethylene producer needed to debottle-

neck its six-stage compressor train to increase

throughout. This project needed to be accom-

plished with minimal cost and downtime.

Using conventional mist eliminators mesh

pads or horizontal ow vanes, the knockout

drum before each compressor stage would have

to be enlarged. For instance, the drum at theinlet to the rst stage, 8 ft in diameter, would be

replaced by a 14 ft vessel.

Amistco thoroughly reviewed the process

conditions and internal geometry of each knock-

out drum. We then customised each existing

drum with double-pocket vanes and mesh

agglomerators, arranged as in Figure 8, using

one, two or four banks, as required. To ensure

proper gas distribution, we added inlet diffusers

as in Figure 11 and ow distribution plates on

the downstream side of the vane units. The result was 35% increased capacity, while achieving an

efciency of 99.9% of 1-micron and larger mist

droplets.

Fertilizer plant capacity boost and amine loss cut

A four-train fertilizer plant needed more ammo-

nia gas processing capacity in one of the trains to

increase production capacity. The bottleneck was

the overhead knockout drum in two of four

carbon dioxide absorber towers in that train.

Those vessels also served as suction drums for the

compressors that followed. In addition, it was

desired to reduce excessive loss of valuable amine

in the form of mist escaping the knockout drums.

Amistco reviewed the absorber process condi-

tions and the available space in the knockout

drums. Although the space in one of the drums

was very tight, we were able to solve this problem

by retrotting both drums with a double-bank

system, as in Figure 8. In each bank, a mesh

agglomerator is followed by an double-pocket

vane unit. The result was a 30% capacity increase.

In addition, the rate of amine loss fell to 0.05

gallon per million standard cubic feet, which

corresponds to savings on the order of $75 000

per year. Another benet was elimination of a

cause of pitting corrosion in the compressors.

Gas production and condensate recovery increase

A large oil and gas company wanted to revampseveral dozen of its 30-year-old two-phase eld

separators (sizes 2–5 ft OD). The purpose was to

increase capacity and improve recovery of

condensate. As in the preceding case, the separa-

tors were also suction drums for compressors.

Joining with a local fabrication shop, Amistco

brought the separators from the eld, removed

the old internals, enlarged the inlet and outlet

nozzles, and retrotted the vessels with Amistco

double-pocket vane units. The separators were

ASME code recertied, painted and reinstalledin the eld with new instrumentation. This reju-

venation saved the company thousands of dollars

per unit as compared to purchasing new separa-

tors. Gas capacity increased by up to 50%, and

condensate recovery went up by many thousands

of barrels per year.

These cases are typical of many applications

where Amistco’s separation technology made a

big difference. Similar results can be achieved

for a wide variety of compressor knockout drums

in reneries, gas plants, oil and gas explorationand production, and petrochemical plants.


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